you can almost do it.
you can add the 'preview in' menu as a drop down to any toolbar. It'll at least save you a couple of clicks and be only one more than a button (without all the cruft).
right click on a toolbar
customize
commands tab
'tools' on the left
select preview in on the right and drag it to a tool bar
For your custom browsers, there's another one in the list on the right there called 'preview in' further down. that one has all the 'additional browsers' in it.
For what it's worth, I don't really use these at all except maybe the first time I open one in a browser because I'm lazy. After that, it's ctrl+s, alt+tab, ctrl+r.
Will